1. Field of the Invention
The present invention relates to a spark plug for use in an internal combustion engine.
2. Description of the Related Art
In a spark plug, according to a widely practiced method for attaching, in a sealed condition, a cylindrical metallic shell to an insulator inserted into the metallic shell, one end portion of the metallic shell is crimped. When the crimping method is to be employed, the configuration of the metallic shell must be determined such that crimping involves neither generation of stress in a portion of the spark plug in which generation of stress is not desirable, nor generation of stress in an undesirable direction. Further, a configuration is desired which prevents unnecessary deformation during crimping to thereby enable stable production of highly accurate metallic shells.
According to popular practice, a tool engagement portion (a so-called hexagonal portion) of a spark plug whose dimensions conform to the industrial standard for engagement with a tool has a dimension of, for example, 16 mm, 19 mm, or 20.8 mm as measured between opposed sides. However, in order to cope with a recent tendency of spark plugs decreasing in size, employment of a tool engagement portion of smaller size (e.g., the distance between opposed sides of a hexagonal portion is 14 mm or less) is seen. When outside dimensions of such a hexagonal portion are determined, the maximum wall thickness of the hexagonal portion is limited in relation to the outside diameter of an insulator (in some cases, the wall thickness becomes insufficient, and as a result the hexagonal portion becomes susceptible to deformation induced by stress).
Therefore, a configuration is desired which enables stable production of highly accurate metallic shells, even when the metallic shells include a portion susceptible to deformation induced by stress as described above.
An object of the present invention is to provide a spark plug having a metallic shell which maintains dimensions at high accuracy and whose crimped portion exhibits high sealing capability.
To achieve the above object, a spark plug of the present invention is characterized in that a cylindrical metallic shell having a tool engagement portion used for mounting the spark plug on an engine is fixedly attached to an axially extending insulator inserted into the metallic shell, by crimping a protrusion formed at one opening portion of the metallic shell toward a crimp rest portion formed on the outer circumferential surface of the insulator to thereby form the protrusion into a crimped portion of the metallic shell, and that the distance between opposed sides of the tool engagement portion is not greater than 14 mm; and the crimped portion as projected orthogonally on a virtual plane in parallel with an axis of the insulator is curved such that an end-side part of the crimped portion approaches the insulator, such that an exterior outline of the crimped portion has an outwardly convex crimped curve portion at the end-side part, and such that a tangent to the exterior outline at a base point of the crimped curve portion and a line perpendicular to the axis projected on the virtual plane form an angle of 50xc2x0-110xc2x0. Preferably, the distance between opposed sides of the tool engagement portion is not less than 10 mm. When the distance is less than 10 mm, the wall thickness of the tool engagement portion becomes insufficient, with a resultant potential failure to maintain required accuracy and strength.
In order for a portion of the metallic shell which is desirably unsusceptible to deformation in the course of crimping to maintain high shape accuracy after crimping, crimping conditions, such as the speed of lowering a crimping punch for pressing down the protrusion to be crimped and the positional relationship between the metallic shell and the crimping punch, are carefully determined. The greater the tolerances of the crimping conditions, the shorter the time required for setting the crimping conditions, thereby contributing to enhancement of yield. According to the above-described configuration, most of a crimping force is imposed in the axial direction of the metallic shell during crimping, and stress generated in the metallic shell in a radial direction is very small. Thus, by imparting at least a certain wall thickness to a portion (e.g., the tool engagement portion) of the metallic shell which is desirably unsusceptible to deformation in the course of crimping, the portion can stably maintain high shape accuracy after crimping. Also, a rather large minus-side tolerance can be employed for the wall thickness of such a portion.
In addition to the above-described configuration, a sealing filler layer may be provided in the gap between the inner surface of the metallic shell and the outer surface of the insulator in a filling condition while being compressed between the crimped portion and the crimp rest portion, to thereby seal the gap. Particularly, when the sealing filler layer is made of talc or the like, by employing the above-mentioned angle condition for the crimped portion of the metallic shell, a portion of the metallic shell which serves as an outer wall for the sealing filler material (hereinafter also called a sealing-filler-layer outer wall portion) can be effectively prevented from deforming in a radial direction; i.e., radially outward swelling of the sealing-filler-layer outer wall portion of the metallic shell can be effectively prevented, whereby a compressive force imposed on the sealing filler layer can be maintained. Thus, the sealing filler layer maintains sufficient density, thereby contributing greatly to prevention of leakage of combustion gas.
Preferably, seal rings are provided at axially opposite sides of the sealing filler layer so as to seal against the insulator and the metallic shell, thereby ensuring sealing effects. In the case of a spark plug employing such seal rings, the sealing filler layer is axially compressed between the seal rings and is thus squeezed out radially outward. Accordingly, the seal rings enhance gastightness but cause imposition of a radially outward load on the sealing-filler-layer outer wall portion of the metallic shell. Therefore, adequate adjustment is desirably carried out so as to prevent deformation of the sealing-filler-layer outer wall portion. Since, as mentioned previously, a radially outward force generated in relation to crimping is decreased, tolerance toward a pressure imposed on the sealing-filler-layer outer wall portion by the sealing filler layer increases. Thus, the sealing filler layer can be compacted while the shape of the sealing-filler-layer outer wall portion is maintained with high accuracy. That is, employing the above-mentioned angle condition is very effective for a spark plug employing the sealing filler layer as well as for a spark plug configured such that the sealing filler layer is compressed between seal rings.